Chemotherapy and radiotherapy kill cancer cells by inducing various types of cell death including apoptosis (Greek for falling to death), which is a natural cell suicide process. Primary cancers often exhibit dramatic initial responses to such therapies. However, most metastatic cancers, such as lung and pancreatic cancers, inevitably recur, leading to treatment failure. Apoptosis is generally assumed to be an intrinsically irreversible process. However we recently discovered an unexpected natural reversibility of execution-stage apoptosis in variety of human cancer cells and mouse primary cells. Dying cells can reverse apoptosis even after caspase-3 activation, which is widely believed to be the point of no return. We named this new recovery process anastasis (Greek for rising to life). Simply removing the apoptosis inducer can allow dying cells to reverse apoptosis, indicating that anastasis is a natural cellular recovery process. Notably, some cells that reverse apoptosis and harbor DNA damage appear to be transformed based on colony formation assays, and they have a higher frequency of chromosome rearrangements, suggesting that anastasis may promote survival of cells with oncogenic potential. Our findings lead to fundamental key questions: can reversal of apoptosis occur in vivo after treatment with apoptosis-inducing cancer therapies, and if so, can anastasis contribute in cancer recurrence after the therapies? In case true, anastasis would be a novel therapeutic target in cancer treatment. Recently, we have successfully developed a highly sensitive in vivo biosensor that allows us to identify cells that have undergone anastasis in Drosophila melanogaster. In Aim 1, we will develop new mammalian versions of the biosensor, and generate transgenic mice for future use to detect anastasis in vivo. In Aim 2, we propose to identify key regulators of anastasis. The proposed work has potential to uncover novel mechanisms of tumor development and evasion, and to provide new insights into the treatment of cancers by targeting anastasis.